Unregistered services refer to services performed instead of a session requested by a calling party, although the calling party requests to establish the session via an IMS domain with a target UE (User Equipment), since the target UE has unregistered in the IMS domain. Here, the term “unregistered” means a state that the registration in the IMS domain has not been done. For example, the unregistered services may include a Call Forward service, a Voice Mailbox service, and the like. These unregistered services are based upon subscription. That is, the unregistered services are performed only after a UE has subscribed the corresponding services.
The subscription based unregistered service under an existing IMS framework was processed according to an iFC (Initial Filter Criteria) in an S-CSCF (Serving-Call Session Control Function). That is, once an IMS UE (User Equipment) (or, IMS terminal) has subscribed an unregistered service, the corresponding information is stored in a HSS (Home Subscriber Server). In particular, the HSS records that the IMS UE has subscribed in the unregistered service and generates the iFC for the IMS UE. Therefore, when the IMS UE, which has subscribed the unregistered service, is in unregistered state to IMS domain, and when the S-CSCF receives a request for establishing a session with the UE, the S-CSCF can receive information (contents) related to the corresponding unregistered service from the HSS to transfer the session to an AS (Application Server).
Hereinafter, how to process a Call Forward among the IMS unregistered services will be described with reference to FIG. 1. The Call Forward is a representative IMS unregistered service. Preconditions as follows may be required in order to execute the service.                UE-B has subscribed a Call Forward service and but is in an unregistered state to an IMS domain.        UE-B has set a Call Forward for a session received from UE-A.        UE-B has set a Call Forward for every session received.        
Upon subscribing the Call Forward service, iFC is installed in the HSS so as to enable performing of a triggering logic for the service.
FIG. 1 is a block diagram illustrating a Call Forwarding process in an IMS domain according to the related art. For the sake of explanation, the components in FIG. 1, such as HSS, I-CSCF and S-CSCF will be briefly described.
First, the HSS is a user information database evolved from HLR. The HSS supports IMS entities performing session control. The HSS stores user-related subscription information related to a multimedia session control, namely, user profile information including user position information, security information for user authentication and user permission and a user-subscribed service. The HSS uses DIAMETER protocols (e.g., Cx, Dx and Sh interfaces).
The CSCF is a type of SIP (Session Initiation Protocol) server which processes a SIP signal in an IMS. The I-CSCF of the CSCFs is located at the boundary of a management domain when no border function exists. The I-CSCF provides an IP address thereof to a DNS of a domain such that a server in another domain finds the I-CSCF to use it as an input node of a network. The I-CSCF uses the DIAMETER (e.g., Cx, Dx and Sh interfaces) to query the HSS as to the user's location and transfers a SIP message to a S-CSCF serving for the user, which is known by the querying.
The S-CSCF is generally one of SIP servers as a central node of a signal layer to perform a session control. The S-CSCF always belongs to a home network. The S-CSCF may download a user profile from the HSS or upload it in the HSS using the DIAMETER (Cx and Dx interfaces). The S-CSCF also binds the user's location and the SIP address, upon the SIP registration by the user. In addition, the S-CSCF is included in a path of every signal message to thusly inspect every message, determines a server to provide a service, routes a SIP message to an application server and performs policies of a network operator.
Referring to FIG. 1 to explain the Call Forwarding process in the IMS according to the related art, UE-A sends an INVITE request to a S-CSCF#1 (S101). The S-CSCF#1 having received the INVITE request performs a service logic to obtain an address of an I-CSCF (Interrogating-Call Session Control Function) allocated thereto (S102). The S-CSCF forwards the INVITE to the I-CSCF with the obtained address (S103).
After receiving the INVITE, the I-CSCF sends a location query for location (position) information on a termination UE to the HSS (S104). The HSS then sends the location information on the termination UE to the I-CSCF in response to the query (S105). The location information may include an address of an S-CSCF to which the INVITE is to be sent. After receiving the response, the I-CSCF sends the INVITE to an S-CSCF#2 using the address received from the HSS (S106).
The S-CSCF#2 receives the INVITE message and then receives a user profile from the HSS. And, the S-CSCF#2 performs a service control based upon an iFC stored in the user profile (S107). The iFC may include several information related to an unregistered service subscribed by the user. The corresponding iFC may also include an address of an AS which is capable of performing the Call Forward. The S-CSCF#2 sends the INVITE message to the AS with the address of the AS included in the iFC (S108).
The AS having received the INVITE performs a service logic for the Call Forward (S109). After performing the Call Forward function, the AS generates a response message indicating that a call should be forwarded to a specific address and sends the generated response to the S-CSCF#2 (S110).
The S-CSCF#2 transfers the response message to the UE-A (S111). The UE-A having received the response message re-sends the INVITE to the specific address included in the response message (S112).
The iFC previously mentioned will now be described in more detail.
The iFC filtering may be used to process an initial SIP request. The S-CSCF may use the iFC in order to determine whether to process the received SIP request. The iFC may be stored in the HSS together with the user profile for management. The S-CSCF may receive the iFC from the HSS when the S-CSCF receives a request for registering in the IMS from an UE or when the S-CSCF receives an INVITE message for the unregistered service. The iFC is available in the S-CSCF for the user's registration period. The iFC may include information as follows.                AS address: This is an AS address to be accessed for a specific subscriber.        Default handling: This includes contents (information) as to whether to give up a trigger having low priority, to open conversation or the like.        Trigger point: This is information received from the HSS and defines SPT for a specific application. The S-CSCF analyzes filter criteria when receiving an initial SIP request. When the initial SIP request is identical to specific filter criteria, the S-CSCF performs a function of delivering (proxying) the request to a predefined SIP AS or the like.        
As described above, in the unregistered service procedure according to the related art, a user performs the subscription to the corresponding service and then information related thereto is stored in the so-called HSS with an IMS framework in a format of iFC. Generally, the S-CSCF receives a user profile by accessing the HSS. That is, the S-CSCF evaluates the iFC when receiving an initial request. Even for this, the corresponding triggering point should be previously installed in the iFC by the registration.
However, since a CSI (combination of CS (Circuit Switching) and IMS (IP Multimedia Subsystem)) is a non-subscription based service, no iFC for the CSI exists separately. Accordingly, in case of a CSI interworking, namely, in case where an IMS UE sends an initial request to a CSI UE, if the CSI UE is in unregistered state to IMS domain, an IMS domain may send a response message such as 400 Fail or the like to the IMS UE.
A detailed example therefor is illustrated in FIG. 2.
FIG. 2 is a block diagram illustrating a process in which an initial request received from an IMS UE is failed since a CSI UE is in an unregistered state.
The procedures illustrated in FIG. 2 are performed as follows.                An IMS UE is in an IMS registered state but a CSI UE is in an IMS unregistered state.        The CSI UE has no subscription for an unregistered service (e.g., Call Forward, Voice Mail Box, etc.).        
As illustrated in FIG. 2, the IMS UE sends an INVITE request for the communication with the CSI UE to the S-CSCF2 (S201).
The S-CSCF2 performs a service logic, and then the S-CSCF#2 acquires an address of an I-CSCF. The S-CSCF2 sends the INVITE to the I-CSCF using the acquired address (S202).
The I-CSCF sends a location query to the HSS in order to obtain location information related to a termination UE (S203). The I-CSCF then receives the location information related to the termination UE from the HSS in response to the location query. The location information may include the address of an S-CSCF to which the INVITE message is to be sent. The I-CSCF sends the INVITE message to the S-CSCF1 by the address included in the information received from the HSS (S205).
The S-CSCF1 receives a user profile from the HSS and performs a service logic based upon the iFC stored in the user profile (S206). The iFC may include several information related to many services subscribed by the user. Here, the iFC indicates that the CSI UE is currently in an IMS unregistered state and has not subscribed an unregistered service. The S-CSCF1 does not find an iFC matched with the initial SIP request received and thus sends a Fail message to the I-CSCF (S207).
The I-CSCF having received the Fail message sends a Fail response message to the S-CSCF2 (S208). The S-CSCF2 then forwards the Fail response message to the IMS UE (S209).
As mentioned above, even if the termination side is the CSI UE, since the CSI UE is in the IMS unregistered state, it fails to set up a session. That is, since the CSI is a non-subscription based service, no method has been provided to send an initial request received from an IMS domain to a CS domain in case of the unregistered state of the CSI UE.